A quantitative analysis of the degeee and extent of coronary arterial narrowing by atherosclerotic plaques in the entire lengths of each of the 4 major coronary arteries in a group of patients dying suddenly from coronary heart disease ("sudden coronary death") is described at necropsy for the first time. Of the 1564 five-mm segments examined in the 31 study patients, 557 (36%) were 76-100% narrowed in cross-sectional area by atherosclerotic plaques (controls equal 3%); 536 (34%) were 51-75% narrowed (controls equal 22%) 360 (23%) were 26-50% (controls equal 42%) and only 111 segments (7%) were less than 25% narrowed (controls equals 33%).